1. Field of the Invention
This invention relates to an ignition timing control system for internal combustion engines, more particularly to an ignition timing control system for vehicle internal combustion engines having a plurality of sets of ignition timing control characteristics corresponding to fuels with different octane numbers that conducts knock control based on a selected characteristics set, and still more particularly to such an ignition timing control system for vehicle internal combustion engines which under specific conditions replaces the selected set of characteristics as a relief measure when the selection has been made based on an erroneous determination.
2. Description of the Prior Art
In response to rising user demand for high power internal combustion engines, automobile manufacturers have in recent years been offering an increasing number of engines designed to use high octane number fuel. In such engines the ignition timing has to be set differently between the case where the fuel actually used has the intended octane number and the case where it does not, since the fuel actually used does not always have the intended octane number. The engine is therefore provided with different sets of ignition timing control characteristics corresponding to different octane numbers. The control system is designed to determine or presume the octane number of the fuel being used from the occurrence state of engine knock and to select one set of control characteristics corresponding to the determined fuel octane number for ignition timing control. Such a system is disclosed, for example, in Japanese Laid-open Patent Publication No. 1(1989)-96473.
Conventional systems provided with a plurality of sets of control characteristics have a drawback, however, in that the octane number of the fuel may be erroneously determined to be lower than the actual octane number, in which case characteristics which retard the ignition timing more than required are selected. As a result, the engine output may be needlessly reduced. It therefore becomes necessary, at a time when the operation condition of the engine is appropriate, to cancel the selected characteristics and select control characteristics for shifting the ignition timing in the advance direction. This thinking is applied, for example, in the invention set out in the assignee's Japanese Laid-open Patent Publication No. 1-224468 (Application No. 63(1988)-51268.) Specifically, the system disclosed in this publication waits until the engine speed has remained below a prescribed level for a prescribed period of time and then forcibly shifts or switches over to control characteristics further in the direction of advanced ignition timing.
With this system, however, when control characteristics are erroneously selected, for example, during vehicle acceleration, shift to appropriate control characteristics is not conducted unless the engine speed falls below the prescribed level. Moreover, it may happen that the control characteristics are shifted (advanced) to those appropriate for a high octane number fuel when in fact the engine is running on low octane number fuel using appropriately retarded control characteristics. The knock which occurs in this case causes the system to shift back to the low octane number characteristics, with the result that "hunting" occurs in which the system switches back and forth between the two sets of characteristics. The control is thus unstable. Further, if the characteristics is easily shifted to the more advanced one, the operator might experience unpleasant feeling owing to the sudden torque change caused by the ignition timing change.